Only an SLF view.
Sorry for butting in guys but as the holder of a VHF radio licence for the marine band I would like to add my tuppenceworth. I was taught by the RNLI that:-
Mayday (3times) should be used to declare an emergency which endangers the vessel and/or its passengers/crew.
Pan Pan (3 times) should be used to indicate an urgent transmission, taking precedence over everything except Mayday calls to report a threat to life or major problem short of a threat to the entire hull. (Note it is NOT a Pan call - it is a Pan-Pan call)
ie,One of 2 engines out in flight is a threat to the hull therefore a Mayday (even if you can continue on one engine since loss of the remaining engine could be a bad thing! just ask Ryanair) a heart attack passenger is regretable but no threat to the hull therefore a Pan Pan call if you want to off load them at the nearest airport.
Of passing interest - until about the mid 80s a man overboard (marine style) was just a Pan Pan and not a Mayday since there was no threat to the hull!!